Astronomers Discover Ancient System with Five Small Planets

This artist's conception illustrates a planet known to comfortably circle in the habitable zone of a sun-like star. A new study estimates that there are billion "alien Earths" in our own Milky Way galaxy.

Kepler-22b - Comfortably Circling within the Habitable Zone

In this handout illustration made available on December 2011 by NASA, a diagram compares our own solar system to a star system containing the first 'habitable zone' planet discovered by NASA's Kepler mission.